If someone were to ask you what is the color of the ocean, chances are that you would answer that is was blue. For most of the world's oceans, your answer would be correct. Pure water is perfectly clear, of course -- but if there is a lot of water, and the water is very deep so that there are no reflections off the sea floor, the water appears as a very dark navy blue. The reason the ocean is blue is due to the absorption and scattering of light. The blue wavelengths of light are scattered, similar to the scattering of blue light in the sky but absorption is a much larger factor than scattering for the clear ocean water. In water, absorption is strong in the red and weak in the blue, thus red light is absorbed quickly in the ocean leaving blue. Almost all sunlight that enters the ocean is absorbed, except very close to the coast. The red, yellow, and green wavelengths of sunlight are absorbed by water molecules in the ocean. When sunlight hits the ocean, some of the light is reflected back directly but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light we see is composed of the shorter wavelength blues and violets.
If there are any particles suspended in the water, they will increase the scattering of light. In coastal areas, runoff from rivers, resuspension of sand and silt from the bottom by tides, waves and storms and a number of other substances can change the color of the near-shore waters. Some types of particles (in particular, the cells of phytoplankton, also referred to as algae) can also contain substances that absorb certain wavelengths of light, which alters its characteristics.
Right Image: Although microscopic, phytoplankton can bloom in such large numbers that they can change the color of the ocean to such a degree that we can measure that change from space. The swirls of green are a phytoplankton bloom in the Gulf of California.
The most important light-absorbing substance in the oceans is chlorophyll, which phytoplankton use to produce carbon by photosynthesis. Due to this green pigment - chlorophyll - phytoplankton preferentially absorb the red and blue portions of the light spectrum (for photosynthesis) and reflect green light. So, the ocean over regions with high concentrations of phytoplankton will appear as certain shades, from blue-green to green, depending upon the type and density of the phytoplankton population there. The basic principle behind the remote sensing of ocean color from space is this: the more phytoplankton in the water, the greener it is....the less phytoplankton, the bluer it is. There are other substances that may be found dissolved in the water that can also absorb light. Since these substances are usually composed of organic carbon, researchers generally refer to these substances as colored dissolved organic matter, CDOM for short.
The study of ocean color helps scientists gain a better understanding of phytoplankton and their impact on the Earth system. These small organisms can affect a system on a very large scale such as climate change. Phytoplankton use carbon dioxide for photosynthesis and in turn provide almost half the oxygen we breathe. The larger the world's phytoplankton population, the more carbon dioxide gets pulled from the atmosphere, hence, the lower the average temperature due to lower volumes of this greenhouse gas. Scientists have found that a given population of phytoplankton can double its numbers about once per day. In other words, phytoplankton respond very rapidly to changes in their environment. Large populations of these organisms, sustained over long periods of time, could significantly lower atmospheric carbon dioxide levels and, in turn, lower average temperatures. Carbon can be 'stored' in oceanic sediments when organic matter sinks and is buried in the ocean floor.
Understanding and monitoring phytoplankton can help scientists study and predict environmental change. Since phytoplankton depend upon sunlight, water, and nutrients to survive, physical or chemical variance in any of these ingredients over time for a given region will affect the phytoplankton concentrations. Phytoplankton populations grow or diminish rapidly in response to changes in its environment. Changes in the trends for a given phytoplankton population, such as its density, distribution, and rate of population growth or diminishment, will alert Earth scientists that environmental conditions are changing there. Then, by comparing these phytoplankton trends to other measurements - such as temperature - scientists can learn more about how phytoplankton may be contributing to, and affected by, climatic and environmental change.
Below are several color samples extracted from this image, with a brief explanation of the likely cause of the dominant color.
Credit for all Patagonian images: Image courtesy Norman Kuring, SeaWiFS Project, image descriptions courtesy of James Acker, NASA GES DISC Oceans Data Team/SSAI.